The present invention relates to magnetic recording media and magnetic recording apparatuses capable of recording information of large capacity and, more particularly, it relates to a magnetic recording medium suitable to high density magnetic recording and a magnetic recording apparatus of small-sized and large recording capacity using the same.
Along with rapid development in the IT industry, a demand for larger recording capacity has been increased more and more for magnetic disk apparatus. To cope with the demand, it has been required to develop magnetic heads with a high degree of sensitivity and recording media with a high S/N ratio. In order to improve the S/N ratio of recording media, it is necessary to improve the read output when recorded at high density. Generally, a recording medium comprises a first underlayer referred to as a seed layer formed on a substrate, a second underlayer of a body-centered structure comprising a Cr alloy, a magnetic layer, and a carbon protection layer. For the magnetic layer, a Co-based alloy of a hexagonal close-packed structure mainly comprising, for example, Co—Cr—Pt—B or Co—Cr—Pt—Ta, is used. In order to improve the read output, it is effective to orient the magnetic layer such that the (11.0) plane or (10.0) plane is substantially parallel with the substrate surface to direct the c-axis as an easy axis of magnetization in the in-plane direction. It has been known that the crystal orientation of the magnetic layer can be controlled by the seed layer. In addition, it has been reported that orientation in the former is obtained by using Ta (JP-A No. 4-188427, JP-A No. 8-212531, U.S. Pat. No. 3,298,893) or MgO (Appl. Phys. Lett., Vol. 67, pp. 3638-3640, December (1993)) and orientation in the latter is obtained by using an NiAl alloy (U.S. Pat. No. 5,693,426) of the B2 structure or the like. Further, it has been known that the read output can be improved also by applying mechanical texturing to the substrate surface to introduce magnetic anisotropy in the circumferential direction. Heretofore, texturing has been applied mainly to an Al—Mg alloy substrate coated with an NiP plated layer. However, JP-A No. 2001-209927 discloses that the magnetic anisotropy can be introduced also by subjecting the surface of a glass substrate to texturing.
On the other hand, a reduction in medium noise is also an important subject for improving the medium S/N ratio, along with an improvement in the read output. In order to reduce medium noise, it is effective to form the magnetic layer from finer particles and decrease Br×t as a product of residual magnetic flux density (Br) and a film thickness (t) of the magnetic layer. However, since excess miniaturization of particles or excess reduction of Br×t deteriorates the thermal stability, a limit is imposed on the noise reduction. In recent years, anti-ferromagnetically coupled medium has been proposed (Appl. Phys. Lett., Vol. 77, pp. 2581-2583, October (2000); Appl. Phys. Lett., Vol. 77, pp. 3806-3808, December (2000)) as a technique for compabilitizing the thermal stability and noise reduction. In this technique, the magnetic layer is formed as a two-layered structure anti-ferromagnetically coupled by way of an Ru intermediate layer, in which Br×t can be set lower while keeping the thickness of the magnetic layer great as it is, compared with a medium comprising a single magnetic layer. Thus, the medium noise can be reduced while keeping the thermal stability.
However, even the combination of the techniques described above is still insufficient to attain an areal recording density of 70 Mbits or more per 1 mm2 and it is further necessary to improve the read output and reduce the medium noise.